Monolithically integrated photonic circuits are useful as optical data links in applications such as, but not limited to, optical communications, high performance computing, and data centers. For mobile computing platforms too, a PIC is a promising I/O for rapidly updating or syncing a mobile device with a host device and/or cloud service where a wireless link has insufficient bandwidth. Such optical links utilize an optical I/O interface that includes an optical transmitter and/or an optical receiver, at least one of which utilizes a photodetector, typically a photodiode.
Avalanche Photodiodes (APD) are useful in applications where high sensitivity is desired. Such applications include long haul fiber-optic telecommunication, laser rangefinder, and single photon level detection and imaging, among other applications. Hybrid silicon/germanium (Si/Ge) APD architectures offer promising applications targeted at near-infrared optical signals. In a Si/Ge Separate Absorption, Charge, and Multiplication (SACM) APD, Ge offers high responsivity at near-infrared wavelengths, while Si is used to amplify photo-generated carriers with low noise. In addition, Si/Ge based APDs have potential to reduce costs compared to APD counterparts in III-V material systems because of Si/Ge APD architectures are more compatible with CMOS technology. Si/Ge APD is therefore a promising candidate for price sensitive markets that require high sensitivity in the near infrared spectrum, such as optical links for server machines and consumer electronics.
High operating biases and high packaging costs remain an obstacle to such PICs however. Si/Ge APDs in the art often require significantly more than 12V and therefore are generally beyond the operating space of server machines, desktop computers, mobile devices, etc. Packaging of an APD can account for 70% of a receiver module. Many such module packaging schemes rely on edge coupling of optical fibers to waveguides leading to the APD. Depending on the dimensions of the waveguides, such active alignment is often inefficient and not amenable to high volume manufacturing.